An electret condenser microphone was developed more than 30 years ago, and is still widely used as a component of a cellular phone and the like.
FIG. 8 is a diagram showing the basic structure of an electret condenser microphone. Referring to FIG. 8, a protective cloth 101 is a cloth which is placed for the purpose of preventing dust or the like from entering, and which is configured by a cloth having a high sound transmission property, such as nonwoven cloth. As a fixed electrode 102, used is a metal electrode or an electrode in which a face opposed to a vibrating diaphragm is electroconductive-treated. An electret film 103 is a dielectric film which is disposed on the fixed electrode 102, and generally used by applying FEP (tetrafluoroethylene-hexafluoropropylene) to the fixed electrode 102 by means of thermal fusion. A thin metal film or a plastic film in which one face is electroconductive-treated is used as the vibrating diaphragm 104. The vibrating diaphragm is bonded to a vibrating diaphragm support ring 105. A spacer 106 is disposed so that the vibrating diaphragm and the fixed electrode form and hold a predetermined positional relationship. Circuit components 107 are configured by an FET, a resistor, etc. On a circuit board 108, the circuit components 107 are mounted by soldering or the like, and form a preamplifier which converts a change of an electrostatic capacitance between the vibrating diaphragm and the fixed electrode due to vibration displacement, to an electric signal, and a terminal board and a bottom face plate are configured. A spacer 109 is used for forming a desired space between the fixed electrode 102 and the circuit board 108. As a case 110, used is a metal such as aluminum, or a molded plastic which is electroconductive-treated. A lower end portion of the case is subjected to a caulking process, bonding, or the like, and the case forms a housing and functions also as a shield case.
The electret condenser microphone basically operates so that an electric potential is produced between the vibrating diaphragm and the fixed electrode, and a displacement of the vibrating diaphragm due to a sound pressure is taken out as an electric output. When charges are produced in an electret layer, a high potential (several tens of volts) can be held between the vibrating diaphragm and the fixed electrode, and a displacement of the vibrating diaphragm due to a sound pressure can be taken out as a large electric output.
Conventionally, an electret condenser microphone has been mounted as an attachment component via a connector or the like on a board mounted in a cellular phone or the like. Recently, a microphone itself must be directly mounted on a board of a cellular phone or the like by using the solder reflow method.
As shown in Patent Reference 1, recently, also a parallel-plate condenser microphone which is produced by using the micromachining technique so as to cope with Pb-free reflow has been proposed.
Patent Reference 1: JP-A-2002-95093
However, it is known that electreted FEP has characteristics that charges easily escape as result of heating.
FIG. 9 shows a temperature profile of reflow which is used in direct mounting on a board of a cellular phone or the like. Recently, Pb-free reflow (reflow using a solder material which does not contain Pb) is desired. FIG. 9 shows a temperature profile of typical Pb-free reflow. In Pb-free reflow, heating for about 10 to 30 sec. is performed at 260 deg. Therefore, a component to be reflowed is required to have thermal resistance which can endure such heating.
However, a usual electret condenser microphone does not have thermal resistance against the Pb-free reflow temperature. FIG. 10 shows experimental results in the case where a surface potential was measured when an electrode in which FEP (thickness: 25 μm) is formed as an electret was heated. In FIG. 10, the abscissa indicates the heating temperature (30 sec.), and the ordinate indicates the surface potential of the electret. FIG. 10 shows that the surface potential, which is −250 V at ordinary temperature, is reduced by about 20% at 200° C., and reduced or eliminated by about 90% at 300° C. The experimental results suggest that a usual ECM cannot cope with Pb-free reflow.
In an electret in which charges escape as result of heating, mounting to a cellular phone is hardly performed by reflow. Recently, as described above, also a parallel-plate condenser microphone which is produced by using the micromachining technique so as to cope with Pb-free reflow has been proposed.
However, a microphone which is produced by using micromachining has a problem in that the cost of the microphone itself is raised because the micromachining technique is a processing method which is expensive in the viewpoint of processing steps.
The invention has been conducted in view of the above-described circumstances. It is an object of the invention to provide an electret condenser microphone which can withstand a high temperature. When a microphone is attached to an application equipment, especially, the microphone may be passed through a reflow solder bath for a short time period. It is an object of the invention to provide a heat-resistant structure which can prevent the function from being impaired by a high temperature in the period.